Rotary drills are commonly used for earth boring operations. Such drills are employed in oil and gas wells where the drill string proceeds to great depths. In such operations a drilling "mud" is typically pumped down the drill string for cooling the drilling cutters. When drilling oil and gas wells it is sometimes desirable to "underream" the hole, that is to enlarge the diameter of the hole being drilled at some point a substantial distance below the surface. To effect this, a variety of underreamers, hole openers or the like have been devised. In such operations the fluid pressure due to the drilling mud can be employed for actuating the underreaming tool. In deep well drilling operations it is typical to withdraw the drill string from the hole and install a suitable underreamer either alone or in series with a conventional pilot drill. When the drill string has been tripped back into the hole, pressure of drilling fluid is applied and through any of a variety of prior mechanisms the cutter arms on the underreamer are urged outwardly for enlarging the selected portion of the hole. After drilling, the cutter arms are retracted and the underreamer withdrawn from the hole. Typically such underreamers are extended whenever drilling fluid pressure is applied, which is at all times when drilling is being conducted.
In mining operations it is typical to use somewhat shorter drill strings which do not extend so deeply into the ground and the drilling fluid is often compressed air instead of a slurry of mud and water. Generally speaking the air pressure is not sufficient for actuating the heavy cutter arms of an underreamer. Underreaming is desirable in many mining operations for enlarging the lower portion of a drill hole so that an adequate quantity of explosives can be inserted for breaking the earth formations and permitting mucking of the fragmented earth.
Since the drill strings are shorter, tool wear does not require round tripping of the drill string during the course of most drilling operations. It is therefore desirable to have a means for underreaming a blast hole without first withdrawing the drill string and installing an underreamer. It is also desirable to have an underreamer that can be actuated by means other than the fluid pressure of drilling fluid. Since the underreamer should be in the drill string during drilling of the pilot hole it is desirable to have means for latching the underreamer so that its cutting arms are in a retracted position during at least a portion of the drilling operations.
An arrangement for latching an underreamer by means of air pressure is described in my U.S. Pat. No. 3,817,339 issued June 18, 1974. As described in that patent it is also desirable that the underreamer have fluid passages whereby a portion of cooling fluid can flow to the underreamer arms while another portion flows through a parallel path to a drilling tool connected beneath the underreamer. Cutter arms in accordance with that patent are further described herein.